Clothing advice for -40 degrees

Hey, does anyone out there have advice for what I should be wearing if I am to be standing around in temperatures of -40°F -48°F or colder (without windchill) for hours, at night? I have some warm fleecy things that have kept me warm-ish to around -30 (effective, with wind-chill), but I was only out for 15-20 minutes at a time. This time around, I'll be standing around at night for what will potentially be long periods of time. For my torso, I was thinking of using the thickest wool base layer I can find, along with warm fleece and a shell. I also have a down vest that I can try to stuff in there.

I have no idea what to do for my legs. Wool base layer, plus... some kind of track pants / guide pants, like what Patagonia offers? Or... full-out down pants, with a shell?

Help me! :)